For some lucky reason I have not had much trouble getting my kids to take any meds. I have always just added the salt to the applesauce. My 5 year old hates when I do it now by surprise, cuz I only do it if she hasn't eaten enough things for me to add extra salt to all day.
But when Jordin was fighting taking her meds for a short period I would squirt it into an empty nipple and she would suck it out. Then I woul exchange for her bottle. Maybe you could try that, just mix the salt with a very small amount of formula and have him suck that out of the nipple then give him his regular bottle. That way it would only be a few sucks, maybe stronger but less.
Hope you find something that works!